Add type `GPUSystem` to determine if a system is stored on the CPU or GPU

[efaulhaber]

I split #493 into multiple smaller parts. This here is the first part.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 69.27%. Comparing base (17ad1e5) to head (1b76c4d). Report is 1 commits behind head on main.

:exclamation: Current head 1b76c4d differs from pull request most recent head 6f1b7d4

Please upload reports for the commit 6f1b7d4 to get more accurate results.

Files	Patch %	Lines
src/schemes/boundary/system.jl	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #532      +/-   ##
==========================================
+ Coverage   68.62%   69.27%   +0.65%     
==========================================
  Files          70       69       -1     
  Lines        4060     3967      -93     
==========================================
- Hits         2786     2748      -38     
+ Misses       1274     1219      -55     

Flag	Coverage Δ
unit	69.27% <80.00%> (+0.65%)	:arrow_up:

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